We all know that in the Microsoft Exchange world DAG stands for “Database Availability Group”.
Database – because on a highly available Exchange 2010 Mailbox server, the database, not the server, is the unit ofavailability and it is the database that can be failed over or switched over between multiple servers within a DAG. This concept is known as database mobility.
Group – because the scope of availability is determined by Mailbox servers in a DAG combined in a failover cluster and working together as a group.
Availability – this word seems to be the least obvious and the most obfuscated term here (and also referred to by both other terms). Ironically, it has a straightforward mathematical definition and plays an important role in understanding overall Exchange design principles.
For more details Exchange Team Blog: